This inventor of the present invention disclosed a flow-injection type piezoelectric sensor, which was allowed for issuance by the Taiwan Patent and Trademark Office and was published in THE TAIWAN PATENT GAZETTE and with the publication number of 280866. This flow-injection type piezoelectric sensor has a planar surface, a depression formed in the planar surface, and a piezoelectric crystal located in the depression such that the piezoelectric crystal is covered in two sides thereof with an upper and a lower metal electrodes. In operation, the piezoelectric crystal is contained in a biosensor device having an oscillation circuit, such that the upper and the lower metal electrodes are connected with the oscillation circuit for detecting the presence of a target molecule in a liquid sample flowing on the surface of the piezoelectric crystal. An improvement in function of the flow-injection type piezoelectric sensor described above is called for.